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3-stage coil gun circuit

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Freelancer1331

Mon Nov 12 2012, 11:53PM

Registered Member #7829 Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16

Hello 4HV,

I am new to the forum and to advanced electronics in general. My buddy and I are working on a 3-stage coil gun concept. We designed the circuit linked below as a proof of concept behind the functionality of the device. Given our lack of experience in the field of circuitry, we welcome any and all comments and critiques. We haven't sorted out the specifics of the design and as such the circuit is left in terms of variables. Additionally, we are working within the constraints of the circuit schematic program, TinyCAD, so some symbols like the fiber optic receiver are incorrect.

Circuit schematic:

Thanks for your input,
Ian

Turkey9

Tue Nov 13 2012, 07:59AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

There are some serious flaws in your circuit. First, you may want to do some research about how transistors really work. As a hint, they need a voltage from the bast to emitter to turn on (in the case of a NPN type). In your circuit, since the section with all the power switching and coils doesn't have a ground connection, the transistors will never see a voltage on the base. Analyze the circuit elements on their own to see if they are really getting the voltages that they need.

On another note, I would strongly suggest you don't use BJTs for your coil switches. They simply can't handle the current needed for a coilgun to perform well. Most beginners use SCRs as they can handle very large currents. If you are set on transistors, consider using IGBTs.

The intended functionality of your circuit looks good; the photogate of one stage triggers the next. You even include what I believe are diodes to suppress the reverse current from the coils. The problem is that they won't work as intended in your circuit; the reverse voltage will still build until things break. They are commonly connected in parallel with the coils to short the current at the source.

If you have any other questions about circuit elements I can probably help you out!

Freelancer1331

Tue Nov 13 2012, 07:16PM

Registered Member #7829 Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16

Turkey9,

I appreciate the critiques regarding the transistors and diodes! I imagine I'll be doing quite a bit of learning as the project continues and will surely be asking more questions about circuit elements.

I'll do background research on SCRs and IGBTs as I am unfamiliar with the devices and will post an updated circuit including one of the two in place of the transistors.

Freelancer1331

Wed Nov 21 2012, 09:06PM

Registered Member #7829 Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16

Apologies for the delay, I finally got around to updating the circuit. We decided on IGBTs for our method of switching and the circuit is still in a concept form as no specifics have been chosen yet. Feel free to comment and critique.

Circuit revision 4:

Thanks

Turkey9

Mon Nov 26 2012, 06:07AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Your switching circuitry looks much better! But the sensing and triggering sections need some help.

First off, the way that you're sensing the laser may work, but would take very careful part selection. Photo diodes produce a small voltage when illuminated. In your circuit, that voltage would add to the source and may bring it above the threshold for your not gate. A better way would be to use the photo diode as the source and amplify that voltage.

Also, I'm not familiar with the flip flop you're using, but it looks like two will be triggered at once with a single photo gate. Why do all the sections need to be connected by those diodes? I don't see a reason.

Freelancer1331

Mon Nov 26 2012, 06:14PM

Registered Member #7829 Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16

Thanks for the tip on the photo diodes. I'll look into using an op-amp to amplify the voltage produced by the photo diodes.

As for the diodes connecting the flip flops, I don't know what I was thinking putting them down there. The idea for the flip flops was to activate individual stages based on projectile position. The trigger engages the first coil and the first photo gate disengages the first coil as the projectile passes through the center and engages the second coil and so on. Since the flip flops would be operated from two places, I need a way to isolate each flip flop from the rest of the circuit so that when I engage the first stage, I don't also engage the final two. Is there a component I can use to isolate the flip flops from each other?

Turkey9

Tue Nov 27 2012, 06:03AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Ah I see. I'm assuming that your photo gates are after the coils, right? Then your circuit almost does what you want it to do. There is a small problem with how the flip flops are triggered, however. For example, in order for the first flip flop to turn off, it needs a second rising edge after the trigger edge. This means that you would have to release the button before the projectile reaches the other photo gate. This would probably be pretty hard to do. Same thing for when the projectile is at a coil transition. There may not be enough time between when one photogate goes low and the other goes high. To fix this, you need a circuit that creates a single fast pulse at the rising edge of a signal. Put this circuit in series with your photogates and trigger. Google 555 one-shot for a good one.

If my assumption that the photogates are after the coil might also pose a problem. For maximum efficiency, the projectile must be the same length of the coil or shorter. If the sensor is after the coil, then it won't be triggered until the projectile center point is after the coil center point. This will cause suck-back which will kill your final velocity. What could fix this is if you put a small plastic spacer on the tip of your projectile so that it will block the photogate before the ferrous part is past the coil center point.

The scheme of triggering that your trying to use is unconventional and that's pretty cool. I like that you're trying something different as that is the only way that our hobby will expand!

Freelancer1331

Tue Nov 27 2012, 07:09AM

Registered Member #7829 Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16

Yes, the photo gates will be at the exit end of the coils so I'll make that more clear in the circuit. I'll look into the 555 one-shot and update the circuit when I understand how to integrate it along with the amplified photo gate circuitry you critiqued earlier.

A plastic spacer is a brilliant idea! Its cheap, easy to make and could be replaceable (I imagine it like a snap/press fit into the projectile) should impact damage it beyond repair.

Thanks for your continued interest and support of our project! Being new to the forum and advanced electronics it helps to have support from those who have already walked this path!

Freelancer1331

Wed Nov 28 2012, 09:12PM

Registered Member #7829 Joined: Sat Nov 03 2012, 08:16PM
Location: Portland, Oregon
Posts: 16

I've done some fiddling around on a circuit simulator I have and came up with the circuit below in an attempt to integrate the 555 one shot and to isolate each stage from simultaneous engagement.

Isolated 555 one shot:

Is this feasible? Or am I getting tripped up by the transistors again?

Thanks!

Turkey9

Thu Nov 29 2012, 05:57AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

That circuit should work just fine. The connection of the transistors is a little unconventional, but should work non-the-less. You need to make sure that your transistor can withstand a negative base voltage as that will occur when the one shot is at a low level. However, you could still use the diodes like your previous circuit in order to isolate the flip flops.

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